The present invention relates to an annular milling cutter for machining teeth of a bevel gear.
Among a various kinds of tools designed to cut a bevel gear, a so-called annular milling cutter is widely used. This cutter includes inner and outer blades 1a and 2a alternately arranged on cutter inner and outer bodies 1 and 2 as shown in FIG. 2. Each of the blades is mounted on the cutter bodies by means of a shim 3, a clamp wedge 4, a clamp 5 and a bolt 6 as shown in FIG. 3, along a circle L with its center on a cutter central axis 0 which the inner body is rotatable about. Describing the gear cutting operation for a bevel gear 7 as shown in FIG. 4, the inner body 1 is rotated about the central axis 0 in the direction of an arrow A. A cutter face Fa of each of the inner blades 1a cuts a back face 7a of every tooth of the bevel gear 7, while a cutter face Fb of each of the outer blades 2a a drive face 7b of every tooth of the bevel gear 7.
FIG. 5 is an enlarged fragmentary view of the inner blade 1a in relation to the adjacent outer blade 2a as viewed radially outwardly from the central axis of rotation 0. Specifically, the blades define a distance H between two points 8a and 8b, namely the point 8a at which the extension of the cutter face Fa intersects the extension of the head face La and the point 8b at which the extension of the cutter face Fb intersects the extension of the head face Lb. This distance H is often called a "point width." The distance H is identical to the width of a space between the adjacent two teeth of a gear generated by cutting operation with these blades. This means that this distance H is determined when the specification of a gear desired is determined. The inner blade 1a overlaps the outer blade 2a, thus providing an overlapping area having a horizontal line segment 10 extending between two points 9a and 9b (see FIG. 5). This overlapping area is needed in order not to leave any cut remainder on the space between the adjacent two teeth of the gear. Touching on the dimensional relations, a blade point width Xa along the head face La of the inner blade 1a is identical to a blade point width Xb along the head face Lb of the outer blade 2a, while a tip radius Ra at the outside corner of the head of the inner blade 1a identical to a tip radius Rb at the outside corner of the head of the outer blade 2a. With the tip radii Ra and Rb, the space between the adjacent two teeth of a bevel gear is rounded to dissipate concentration of notch stress when the gear is subject to load.
With the known milling cutter as mentioned above, however, it is impossible to provide a desired rounded profile on each of fillet portions formed adjacent the root portion of each of teeth of a bevel gear since the blade point widths Xa and Xb are identical to each other and the tip radii Ra and Rb are identical to each other, and thus, with the same point width H and the same overlapping area, it is impossble to increase the tip radii Ra and Rb to desired values.
Referring to FIG. 4, the operating life of the bevel gear 7 is limited by the strength of a weaker one of the drive face 7b and the back face 7a since the use of the bevel gear in the forward rotation is more frequent than its use in the reverse rotation, the drive and back faces 7b and 7a are subject to different amounts of load when the bevel gear 7 is in use, and the strengths required on the bevel gear in the forward and reverse rotations, which are determined by the amounts of input load, are different. Accordingly, if, with the strength of that face (the back face 7a in this example) which is less frequently used being reduced and thus somewhat sacrificed, the strength of the other face is increased, the operating life of the bevel gear will increasee. Among many measures to increase the strength of one face of every tooth of the bevel gear, it is found to be most effective to increase the radius of curvature of the rounded fillet portion adjacent the one face of the tooth. However, with the known cutter as shown in FIG. 5, an allowable range which the radius Rb of the outer blade 2a is increased is very small since the ratio of the overlapping area to the point width H cannot be varied. Thus, with the known annular cutter, a bevel gear with an increased operating life cannot be generated. It is, therefore, an object of the present invention to provide an annular milling cutter which can generate a bevel gear with an increased operating life.